Polymeric flocculation: Effects of chemical and physical variables on floc formation and growth

Flocculation of fine-mineral suspensions was studied in an agitated batch system. Systematic studies of various flocculation processes were carried out in a standard mixing tank. The preliminary studies included hydrophobic flocculation of hematite suspensions, sweep flocculation of kaolin suspensions, and polymeric flocculation of alumina suspensions. Floc size distribution and its derivatives obtained from a light scattering technique were primarily used as a direct representation of these processes and as flocculation performance criteria in this study.; The specific emphasis of this study was on polymeric flocculation. Polymers with various degrees of molecular weight and ionicity were used. The viscosity behavior of these polymer solutions under a variety of solution chemistry was evaluated in terms of hydrodynamic and electroviscous effects. The effects of molecular weigh and/or ionicity of the polymers on flocculation of initially stable and unstable suspensions were investigated. It was found that polymers of low molecular weight with high charge density (polymeric coagulants) are very effective for destabilizing stable suspension. On the other hand, polymers with high molecular weight (polymeric flocculants) are very effective for promoting floc growth. Flocculation of initially unstable suspensions leads to the conclusion that the higher the molecular weight or ionicity of the polymers, the larger the hydrodynamic sizes of the polymers, and then the larger the median floc sizes.; Application of reagent combinations in polymeric flocculation leads to effective flocculation. Effective flocculation requires destabilization of the suspension by the addition of acid/base, inorganic salts or polymeric coagulants prior to enhancement of floc growth by the flocculant addition. This order of addition is extremely important in applying the reagent combination scheme.; The effects of physical variables: solids concentration, polymer addition rate, and excess agitation time and intensity on polymeric flocculation of initially unstable suspensions with broad ranges of solids concentration were studied. The effects of these variables were shown to be in general agreement with previous studies especially at high solids concentration. Flocculation at high solids content is controlled by the amount of polymer available in the suspension. On the other hand, the limited flocculation also occurs at low solids content due to steric stabilization.; Simplified models for polymeric flocculation of dilute and concentrated suspensions were separately developed. For the dilute suspension, floc growth is limited by steric stabilization when the suspensions are subjected to overdosing. The simultaneous floc growth and breakage model is appropriate for describing the flocculation kinetics of the concentrated suspension. Both models were shown to be consistent with experimental results.